1. Field of the Invention
The present invention relates to a control device for an automatic transmission and a method of controlling the automatic transmission. Specifically, the present invention relates to a control device for an automatic transmission and a method of controlling the automatic transmission that optimally adjust the clearance between friction engagement elements, when the friction engagement elements are disengaged.
2. Description of the Related Art
Conventionally, in automatic transmissions that includes a torque converter and a planetary gear mechanism, a friction engagement element of multiple-plate clutch type is often used as a clutch or a brake provided to a planetary gear mechanism.
Automatic transmissions that enable a multi-speed gearshift of 6-speed forward drive gears with clutches have been described (for example, refer to Japanese Patent Application Publication No. 2003-240068 (JP-A-2003-240068)). Among the friction engagement elements of such automatic transmissions, a second brake may be engaged by moving a piston to the axial direction, with the aid of hydraulic pressure, to press the clutch plates against each other. The clutch plates are constituted by a rotating friction plate that has a friction member attached on both surfaces and by a stationary friction plate that is fixed to a case. The second brake may be disengaged through the urging force of a return spring.
The second brake, which establishes a first gear step, includes parallel one-way clutches. Thus, the second brake is not engaged when the vehicle starts moving, but the second brake is engaged when engine brake is applied.
However, in the conventional automatic transmission described in JP-A-2003-240068, for example, when the vehicle is moving at high speed, the rotating friction plate runs idle at high speed in relation to the stationary friction plate, according to the speed of the vehicle.
When the rotating friction plate rotates at high speed and the clearance between the friction member of the rotating friction plate and the stationary friction plate, that is the pack clearance, is relatively large, the rotating friction plate moves intensely in relation to the stationary side friction plate, and the friction member of the rotating friction plate comes in contact with the stationary friction plate. Accordingly, friction force increases, and drag torque of the clutch plate increases.
As a result, the oil temperature in the automatic transmission increases and may have a tendency to easily overheat.
However, it is possible to reduce pack clearance in order to prevent intense motion of the rotating friction plate in a high-speed driving range.
However, if the rotating friction plate rotates at a rotational speed in a normal-speed driving range when the clearance is reduced, oil tends to be pooled between the friction member and the stationary friction plate, compared to a case where the rotating friction plate rotates at the rotational speed in a high-speed driving range. Therefore, the shear force of oil increases, and accordingly drag torque of the clutch plate is increased.
In addition, it also causes a problem that, when the drag torque of the clutch plate increases, power loss increases and accordingly the fuel economy of the vehicle decreases.